The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the acquisition of MRI data with pulse sequences which sample k-space with interleaved non-rectilinear readout patterns such as spirals, rosettes and radial projections.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Many different pulse sequences are known in the art for acquiring NMR signals from which an image may be reconstructed. Most of these pulse sequences sample k-space in a rectilinear pattern, but there is a class of pulse sequences which sample k-space in non-rectilinear patterns. As disclosed in U.S. Pat. Nos. 5,604,435; RE 32,712, for example, a spiral sampling pattern is achieved by applying a sinusoidally varying readout magnetic field gradient during acquisition of each NMR signal. Similarly, the readout magnetic field gradient can be varied to sample a rosette pattern in k-space. When these non-rectilinear pulse sequences are employed to sample k-space in a set of interleaved patterns, the readout gradients are different at the end of each interleave. Rewinder gradient pulses are often used at the end of such pulse sequences to return the spin magnetization to the center of k-space and the size of these will vary from one interleave to the next.
Severe image artifacts can be produced when using interleaved spiral scans on certain MRI systems. The problem was observed to be greater when the TE period of the pulse sequence was very long (e.g. 30 to 100 msec.). Such pulse sequences are used, for example, when acquiring functional MRI data from the brain of a patient.